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The most recent eruptive period of Tungurahua volcano lasted 17 yr (1999–2016), generating strong eruptive phases with the release of large amounts of seismic and acoustic energies. We have selected 10 large eruptions and located their seismic sources. The location method involves applying a new method for correcting for site effects in the seismic records and identifying, in the frequency domain, the signals from the ground-coupled airwaves and pyroclastic flows. The locations are computed by minimizing the differences of the source energy rates in the [0.4, 2.5] Hz range. The results suggest that the first three eruptions have locations mainly concentrated at the [−1, 2] km depth range (asl), and the locations of the fourth eruption are mainly coincident with depths of a possible reservoir estimated from petrological studies. This fact strongly suggests that this eruption was responsible of changes in the dynamics of Tungurahua, making the volcano prone to produce Vulcanian eruptions. [ABSTRACT FROM AUTHOR]

Quito, the capital of Ecuador, with more than 2.5 M inhabitants, is exposed to a high seismic hazard due to its proximity to the Pacific subduction zone and active crustal faults, both capable of generating significant earthquakes. Furthermore, the city is located in an intermontane piggy-back basin prone to seismic wave amplification. To understand the basin's seismic response and characterize its geological structure, 20 broad and medium frequency band seismic stations were deployed in Quito's urban area between May 2016 and July 2018 that continuously recorded ambient seismic noise. We first compute horizontal-to-vertical spectral ratios to determine the resonant frequency distribution in the entire basin. Secondly, we cross-correlate seismic stations operating simultaneously to retrieve interstations surface-wave Green's functions in the frequency range of 0.1–2 Hz. We find that Love waves travelling in the basin's longitudinal direction (NNE–SSW) show much clearer correlograms than those from Rayleigh waves. We then compute Love wave phase-velocity dispersion curves and invert them in conjunction with the HVSR curves to obtain shear-wave velocity profiles throughout the city. The inversions highlight a clear difference in the basin's structure between its northern and southern parts. In the centre and northern areas, the estimated basin depth and mean shear-wave velocity are about 200 m and 1800 ms−1, respectively, showing resonance frequency values between 0.6 and 0.7 Hz. On the contrary, the basement's depth and shear-wave velocity in the southern part are about 900 m and 2500 ms−1, having a low resonance frequency value of around 0.3 Hz. This difference in structure between the centre-north and the south of the basin explains the spatial distribution of low-frequency seismic amplifications observed during the M w 7.8 Pedernales earthquake in April 2016 in Quito. [ABSTRACT FROM AUTHOR]

The Ecuadorian forearc is a complex region of accreted terranes with a history of large megathrust earthquakes. Most recently, a M w 7.8 megathrust earthquake ruptured the plate boundary offshore of Pedernales, Ecuador on 16 April 2016. Following this event, an international collaboration arranged by the Instituto Geofisico at the Escuela Politécnica Nacional mobilized a rapid deployment of 65 seismic instruments along the Ecuadorian forearc. We combine this new seismic data set with 14 permanent stations from the Ecuadorian national network to better understand how variations in crustal structure relate to regional seismic hazards along the margin. Here, we present receiver function adaptive common conversion point stacks and a shear velocity model derived from the joint inversion of receiver functions and surface wave dispersion data obtained through ambient noise cross-correlations for the upper 50 km of the forearc. Beneath the forearc crust, we observe an eastward dipping slow velocity anomaly we interpret as subducting oceanic crust, which shallows near the projected centre of the subducting Carnegie Ridge. We also observe a strong shallow positive conversion in the Ecuadorian forearc near the Borbon Basin indicating a major discontinuity at a depth of ∼7 km. This conversion is not ubiquitous and may be the top of the accreted terranes. We also observe significant north–south changes in shear wave velocity. The velocity changes indicate variations in the accreted terranes and may indicate an increased amount of hydration beneath the Manabí Basin. This change in structure also correlates geographically with the southern rupture limit of multiple high magnitude megathrust earthquakes. The earthquake record along the Ecuadorian trench shows that no event with a M w >7.4 has ruptured south of ∼0.5°S in southern Ecuador or northern Peru. Our observations, along with previous studies, suggest that variations in the forearc crustal structure and subducting oceanic crust may influance the occurrence and spatial distribution of high magnitude seismicity in the region. [ABSTRACT FROM AUTHOR]

The Ecuadorian convergent margin has experienced many large mega-thrust earthquakes in the past century, beginning with a 1906 event that propagated along as much as 500 km of the plate interface. Many subsections of the 1906 rupture area have subsequently produced M w ≥ 7.7 events, culminating in the 16 April 2016, M w 7.8 Pedernales earthquake. Interestingly, no large historic events M w ≥ 7.7 appear to have propagated southward of ∼1°S, which coincides with the subduction of the Carnegie Ridge. We combine data from temporary seismic stations deployed following the Pedernales earthquake with data recorded by the permanent stations of the Ecuadorian national seismic network to discern the velocity structure of the Ecuadorian forearc and Cordillera using ambient noise tomography. Ambient noise tomography extracts Vsv information from the ambient noise wavefield and provides detailed constraints on velocity structures in the crust and upper mantle. In the upper 10 km of the Ecuadorian forearc, we see evidence of the deepest portions of the sedimentary basins in the region, the Progreso and Manabí basins. At depths below 30 km, we observe a sharp delineation between accreted fast forearc terranes and the thick crust of the Ecuadorian Andes. At depths ∼20 km, we see a strong fast velocity anomaly that coincides with the subducting Carnegie Ridge as well as the southern boundary of large mega-thrust earthquakes. Our observations raise the possibility that upper-plate structure, in addition to the subducting Carnegie Ridge, plays a role in the large event segmentation seen along the Ecuadorian margin. [ABSTRACT FROM AUTHOR]

Quito, the capital city of Ecuador hosting ∼2 million inhabitants, lies on the hanging wall of a ∼60-km-long reverse fault offsetting the Inter-Andean Valley in the northern Andes. Such an active fault poses a significant risk, enhanced by the high density of population and overall poor building construction quality. Here, we constrain the present-day strain accumulation associated with the Quito fault with new Global Positioning System (GPS) data and Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) analysis. Far field GPS data indicate 3–5 mm yr–1 of horizontal shortening accommodated across the fault system. In the central segment of the fault, both GPS and PS-InSAR results highlight a sharp velocity gradient, which attests for creep taking place along the shallowest portion of the fault. Smoother velocity gradients observed along the other segments indicate that the amount of shallow creep decreases north and south of the central segment. 2-D elastic models using GPS horizontal velocity indicate very shallow (<1 km) locking depth for the central segment, increasing to a few kilometres south and north of it. Including InSAR results in the inversion requires locking to vary both along dip and along strike. 3-D spatially variable locking models show that shallow creep occurs along the central 20-km-long segment. North and south of the central segment, the interseismic coupling is less resolved, and data still allows significant slip deficit to accumulate. Using the interseismic moment deficit buildup resulting from our inversions and the seismicity rate, we estimate recurrence time for magnitude 6.5 + earthquake to be between 200 and 1200 yr. Finally, PS-InSAR time-series identify a 2 cm transient deformation that occurred on a secondary thrust, east of the main Quito fault between 1995 and 1997. [ABSTRACT FROM AUTHOR]

The megathrust earthquake that occurred along the Nazca Plate, near the coasts of Colombia and Ecuador in 1906, induced a large tsunami that propagated over the Pacific Ocean. The tsunami arrived in countries very far from the source region, including the United States and Japan. Part of the land covered by railroad tracks in Hilo Bay, Hawaii, United States, was inundated during the tsunami. The tsunami reached a height of several tens of centimetres in other regions of Hawaii, while residents in Hilo witnessed a height of 3.6 m in the bay. This study aimed to estimate the large amplification and inundation mechanisms of the 1906 Colombia–Ecuador tsunami in Hilo Bay, based on a detailed numerical simulation. First, available tsunami observation data were carefully examined and compared with the results of previous studies. The results indicate a maximum inundation height due to the tsunami in Hilo Bay ranging from 1.1 to 1.8 m. Next, a tsunami inundation simulation was performed under varying conditions, such as energy loss by bottom friction and tide level change, using an earthquake source model. The simulated results were consistent with the observation in terms of the inundation height and indicate that bay-scale resonance occurred in Hilo Bay. Furthermore, waves amplified by resonance were more significant than the primary wave. It can therefore be concluded that bay-scale resonance enhanced the tsunami inundation in Hilo Bay. These new results show that the tsunami magnitude of 8.7, estimated in a previous study, is an overestimation. Instead, the results suggest a lower tsunami magnitude of 8.4, which is consistent with the moment magnitude obtained from the earthquake source model. [ABSTRACT FROM AUTHOR]

Scattering and refractions that occur in the heterogenous near-surface beneath seismic stations can strongly affect the relative amplitudes recorded by three-component seismometers. Using data from Tungurahua volcanowe have developed a procedure to correct these 'site effects'.We show that seismic noise signals store site information, and then use their normalized spectral amplitudes as site frequency response functions. The process does not require a reference station (as per the S-wave and coda methods) or assume that the vertical amplitude is constant (the H/V component ratio method). Correcting the site effects has three consequences on data analysis: (1) improvement of the seismic source location and its energy estimation; (2) identification of a strong influence on the volcanic acoustic seismic ratio (VASR) and (3) decoupling the air wave impact on the ground caused by explosions or eruption jets. We show how site effect corrections improve the analysis of an eruption jet on 2006 July 14-15, appearing two periods of strong acoustic energy release and a progressive increase of the seismic energy, reaching the maximum before finishing the eruption. [ABSTRACT FROM AUTHOR]

To improve earthquake location, we create a 3-D a priori P-wave velocity model (3-DVM) that approximates the large velocity variations of the Ecuadorian subduction system. The 3-DVM is constructed from the integration of geophysical and geological data that depend on the structural geometry and velocity properties of the crust and the upper mantle. In addition, specific station selection is carried out to compensate for the high station density on the Andean Chain. 3-D synthetic experiments are then designed to evaluate the network capacity to recover the event position using only P arrivals and the MAXI technique. Three synthetic earthquake location experiments are proposed: (1) noise-free and (2) noisy arrivals used in the 3-DVM, and (3) noise-free arrivals used in a 1-DVM. Synthetic results indicate that, under the best conditions (exact arrival data set and 3-DVM), the spatiotemporal configuration of the Ecuadorian network can accurately locate 70 per cent of events in the frontal part of the subduction zone (average azimuthal gap is 289° ± 44°). Noisy P arrivals (up to ± 0.3 s) can accurately located 50 per cent of earthquakes. Processing earthquake location within a 1-DVM almost never allows accurate hypocentre position for offshore earthquakes (15 per cent), which highlights the role of using a 3-DVM in subduction zone. For the application to real data, the seismicity distribution from the 3-D-MAXI catalogue is also compared to the determinations obtained in a 1-D-layered VM. In addition to good-quality location uncertainties, the clustering and the depth distribution confirm the 3-D-MAXI catalogue reliability. The pattern of the seismicity distribution (a 13 yr record during the inter-seismic period of the seismic cycle) is compared to the pattern of rupture zone and asperity of the Mw = 7.9 1942 and the Mw = 7.7 1958 events (the Mw = 8.8 1906 asperity patch is not defined). We observe that the nucleation of 1942, 1958 and 1906 events coincides with areas of positive Simple Bouguer anomalies and areas where marine terraces are still preserved on the coastal morphology. From north to south: (1) the 1958 rupture zone is almost aseismic and is attributed to a zone of high coupling; (2) south of the Galera alignment (perpendicular to the trench), the 1942 rupture zone presents moderate seismicity, deeper on the seismogenic interplate zone, and abutting on the Jama cluster (to the south). This cluster is facing the Cabo Pasado cap and positive Bouguer anomalies on the overriding margin. We suspect that this cluster reflects a zone of local asperity (partial coupling). South of the Jama cluster, the spherical aseismic zone in the Bahia area is interpreted as having a low seismic coupling (steady creep motion or slow slip events). We suspect that the site that generated the three M > 7 events (1896, 1956 and 1998) correspond to a small patch of strong coupling. To the south, in the Manta-Puerto Lopez zone, the seismicity is mainly organized in earthquake swarms (1998, 2002, 2005). Although slow slip events have been observed in the area (Vallée et al. submitted), we infer from the coastline shape, the marine terraces and the high positive Bouguer anomalies that the seismicity here might reveal a significant amount of seismic coupling. [ABSTRACT FROM AUTHOR]

The whole territory of Ecuador is exposed to seismic hazard. Great earthquakes can occur in the subduction zone (e.g. Esmeraldas, 1906, Mw 8.8), whereas lower magnitude but shallower and potentially more destructive earthquakes can occur in the highlands. This study focuses on the historical crustal earthquakes of the Andean Cordillera. Several large cities are located in the Interandean Valley, among them Quito, the capital (∼2.5 millions inhabitants). A total population of ∼6 millions inhabitants currently live in the highlands, raising the seismic risk. At present, precise instrumental data for the Ecuadorian territory is not available for periods earlier than 1990 (beginning date of the revised instrumental Ecuadorian seismic catalogue); therefore historical data are of utmost importance for assessing seismic hazard. In this study, the Bakun & Wentworth method is applied in order to determine magnitudes, locations, and associated uncertainties for historical earthquakes of the Sierra over the period 1587–1976. An intensity-magnitude equation is derived from the four most reliable instrumental earthquakes ( Mw between 5.3 and 7.1). Intensity data available per historical earthquake vary between 10 (Quito, 1587, Intensity ≥VI) and 117 (Riobamba, 1797, Intensity ≥III). The bootstrap resampling technique is coupled to the B&W method for deriving geographical confidence contours for the intensity centre depending on the data set of each earthquake, as well as confidence intervals for the magnitude. The extension of the area delineating the intensity centre location at the 67 per cent confidence level (±1σ) depends on the amount of intensity data, on their internal coherence, on the number of intensity degrees available, and on their spatial distribution. Special attention is dedicated to the few earthquakes described by intensities reaching IX, X and XI degrees. Twenty-five events are studied, and nineteen new epicentral locations are obtained, yielding equivalent moment magnitudes between 5.0 and 7.6. Large earthquakes seem to be related to strike slip faults between the North Andean Block and stable South America to the east, while moderate earthquakes ( Mw≤ 6) seem to be associated with to thrust faults located on the western internal slopes of the Interandean Valley. [ABSTRACT FROM AUTHOR]